Fluid flow control method and apparatus for minimizing particle contamination

ABSTRACT

A fluid flow control for use with a process chamber. In the disclosed embodiment, the process chamber is for ion implantation of a workpiece and the fluid flow control is to assure the flow rates are maintained at values which are efficient in evacuating and pressurizing the chamber but are not high enough to dislodge particulate contaminants from the process chamber walls. In the disclosed design, the invention has utility both in instances in which wafers are directly inserted into the process chamber for ion implantation and in which the wafers are inserted into the chamber by use of a load-lock which avoids the requirement that the process chamber be cyclicly pressurized and depressurized.

This application is a continuation of application Ser. No. 07/727,828,filed Jul. 9, 1991, now abandoned, which is a division of copendingapplication Ser. No. 07/585,914, filed on May 21, 1990, now U.S. Pat.No. 5,031,674, which is a division of application Ser. No. 07/319,257,filed on Mar. 3, 1989, now U.S. Pat. No. 4,987,933.

TECHNICAL FIELD

The present invention concerns method and apparatus for controlling theflow rates used in evacuating and repressurizing a process chamber insuch a way as to minimize particle contamination on the workpiece.

Background Art

One example of a manufacturing process that requires controlledevacuation and repressurization of a work station is the process ofcontrolled doping of semi-conductor wafers with ions in an ionimplantation chamber. Ions from a source are accelerated along an iontravel path to impinge upon the wafers and introduce controlled doses ofimpurities into the silicon wafers. The ion travel path must beevacuated to assure the ions are well collimated. To accomplish thisprocess in the prior art, wafers have been introduced to an ionimplantation chamber, either through a load-lock or by introducing thewafers directly into the implantation chamber. If a load-lockarrangement is used, the load-lock chamber is successively evacuated andpressurized as wafers are inserted into the load-lock on their travelpath to the ion implantation chamber. If no load-lock is used, thewafers are inserted directly into the ion implantation chamber whichitself must be pressurized, evacuated, and then repressurized as theworkpiece are inserted into the chamber, treated and then removed.

Other examples of processes involving pressurization anddepressurization of a chamber are known in the prior art. In a sputtercoating procedure, for example, a workpiece is inserted into a treatmentchamber and then a coating is applied to the surface of the work pieceby sputtering the coating material away from a target. This procedurecan be used, for example, in coating magnetic material into a recordingmedium. Again, prior to conducting the coating process, the work piecemust be inserted into the chamber and then the coating process conductedat a reduced pressure.

It is often a requirement in these processes that the level ofcontaminants within the processing chamber is kept at a minimum. If thecontaminant level in a doping chamber, for example, exceeds a specifiedvalue, the semi-conductor yield of the process will be reduced.

Although steps are taken to reduce the level of particulatecontamination within a processing chamber, these steps cannot totallyavoid such contaminants. Particulates are inevitably introduced, forexample, as the workpieces are inserted into an ion implantationchamber. These particulates tend to settle on the interior walls of thechamber and remain in place until air flow that occurs during chamberevacuation and repressurization dislodges the particles causing them tomove within the chamber. If the particulate contaminants remain attachedto the chamber walls, the work piece can be inserted into the chamber,treated, and removed without undue contamination. It is when theparticles are dislodged and come to rest on the workpiece either beforeor after the ion implantation process that the particles' presencereduces production yield.

Prior are Stoltenberg U.S. Pat. No. 4,739,787 which issued Apr. 26, 1988recognizes the possibility for contaminant presence affecting the yieldsduring semi-conductor wafer fabrication. This patent recognizes thepossibility of dislodging contaminants from chamber walls as air entersand exits the process chamber. As a proposed solution to the dislodgingproblem, the '787 patent recommends the use of "soft-start valves" whichopen in accordance with a controlled profile so tat "turn on" turbulenceis reduced. Specifically, the '787 patent calls for a pressurization anddepressurization of a chamber in accordance with a time profile.

While the '787 patent recognizes the importance of reducing detachmentof particles from the chamber wall, this patent makes no mention ofcontrolling flow velocity. Instead, the patent focuses on avoidingturbulence and accomplishes this by pressurizing and depressurizing thechamber in a timed sequence which avoids gas flow turbulence. Thepresent invention concerns a type of pressure control for thepressurizing and depressurizing of a chamber to achieve appropriatepressures in an efficient manner without dislodging undue amounts ofcontaminants from the chamber wall.

DISCLOSURE OF THE INVENTION

The present invention concerns method and apparatus for evacuating aprocess chamber and then repressurizing it by pumping air from thechamber and then allowing air to re-enter the chamber. The processcontrols the flow rates of the air as it enters and exits the chamber tominimize dislodging of particular contamination from the chamber walls.

A system for treating one or more workpieces constructed in accordancewith the invention includes a chamber having a chamber interior intowhich the one or more workpiece are moved during the treatment process.The chamber has one or more workpiece openings for inserting thoseworkpiece into the chamber and removing the workpiece after they havebeen treated. The chamber also includes one or more openings forallowing air to enter the chamber through an inlet passageway andevacuating the chamber by withdrawing air in the chamber through asecond outlet passageway. A pressure sensor monitors pressure inside thechamber and provides a pressure signal indicative of the sensedpressure. A programmable controller monitors the pressure signal andadjusts the air flow rates of air entering or exiting the chamber toavoid contamination of the chamber interior caused by too high a flowrate of the air as it moves into and out of the chamber.

A preferred application of the system is for ion implantation of siliconwafers. In this application, the chamber having the openings forinserting the workpiece can either be the process chamber itself inwhich the workpiece is positioned during ion implantation, oralternately, the chamber is a load-lock into which the workpiece isinserted. In this latter application, the workpiece is inserted into theload-lock, the load lock is then depressurized and then the workpiece isagain transferred from the load-lock into a process chamber.

In either application, the flow controller monitors pressure within thechamber and adjusts flow rate of air entering and exiting the chamber toavoid particulate contamination of the chamber. This is most preferablyaccomplished with a pressure sensor and a flow control valve which canbe adjusted based upon the sensed pressure within the chamber to providea specific air flow rate. The correlation between chamber pressure andflow rate is most preferably accomplished with a programmable controllerhaving a look-up table for comparing the sensed pressure within thechamber with a maximum flow rate and outputting a signal to adjust aflow control valve setting to achieve the requisite flow rate. In thepreferred design, a safety factor is built into the look-up table of theprogrammable controller so that a flow rate is produced which shoulddislodge even fewer particles than an amount deemed acceptable.

In an alternate arrangement, the flow rate of air entering and exitingthe chamber can be monitored with multiple pressure sensors and thesetting of an adjustable valve changed in response to the multiplepressure readings from the pressure sensors to provide a desired airflow rate. This technique utilizes more pressure sensors in differentlocations but can be implemented with a less expensive valvearrangement.

From the above it is appreciated that one object of the invention is anew and improved flow control mechanism for use in a process controlthat involves successively evacuating and repressurizing a processchamber. The particular technique disclosed focuses on the actual flowrates rather than the time period in which the depressurization andrepressurization is accomplished. This and other objects, advantages andfeatures of the invention will become better understood from thefollowing detailed description of a preferred embodiment of theinvention which is described in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic description of an ion implantation system;

FIG. 2 is an enlarged elevation schematic showing an ion implantationchamber and load-lock for inserting and withdrawing silicon wafers fromthe implantation chamber;

FIG. 3 depicts an alternate arrangement in which the ion implantationchamber itself is evacuated and then repressurized as wafers are treatedand then withdrawn from the ion implantation chamber;

FIG. 4 is a schematic showing one process control technique forcontrolling the flow rates of air passing through a passageway in fluidcommunication with a chamber;

FIG. 5 is a schematic of an alternate arrangement to the FIG. 4schematic showing a different process for controlling flow rates in anair flow passageway;

FIG. 6 is a graph depicting a look-up mechanism for correlating sensedpressure with maximum allowable velocity of air entering the chamber toavoid undue particle contamination of the chamber.

BEST MODE FOR CARRYING OUT THE INVENTION

Turning now the drawings, FIG. 1 depicts an ion implantation system 10having an ion source 12 and an analyzing magnet 14 contained within afirst housing 16. An ion beam 20 from the source 12 is directed along atravel path which causes it to exit the housing 16 and travel to an ionimplantation chamber 22 positioned inside a second housing 24. Theanalyzing magnet 14 resolves the ions within the beam 20 to produce awell defined beam of a chosen ion species that exits the housing 16 andpasses through a coupling conduit 26 to enter the second housing 24.

The ion implantation chamber 22 is supported on a moveable pedestal 28which facilitates alignment of the chamber 22 in relation to the ionbeam 20. As seen most clearly in FIG. 2, the ion beam 20 enters theimplantation chamber 22 and impinges upon a wafer support 40 that movesindividual silicon wafers along a circular path designed to cause theion beam 20 to impact the wafers and selectively dope those wafers within impurities. The support 40 is mounted for rotation about an axis 42and high speed rotation of the support 40 is accomplished by a motor 40which rotates the support 40 once wafers have been mounted about anouter periphery of the support 40.

The wafer support 40 is mounted for translational movement within thechamber 30 to shift the wafer position back and forth in a scanningmotion that accomplishes a controlled doping of the ion impurities. Thisback and forth translational movement is accomplished by a second motor52 coupled to the support 40 by a drive shaft 54. The motor 50 and wafersupport 40 are supported by a guide rail 56. Two bearings 58 support thewafer support 40 and motor 40 for sliding movement as indicated by thearrow 60 in FIG. 2. During ion implantation of the silicon wafers, thesupport 40 rotates at high speed while moving back and forth asindicated by the arrow 60 to assure a controlled concentration of dopingimpurities impact the wafers mounted about the periphery of the support40.

Semiconductor wafers are inserted into the ion implantation chamber 22through a load-lock 70. the load-lock 70 defines a chamber 72 that isevacuated by a vacuum pump (not shown) to the same pressure as thepressure inside the ion implantation chamber 22 to allow wavers to betransferred back and forth through an opening 74 between the load-lockand the chamber 22.

Automatic mechanisms for moving the wafers into and out of the chamber22 are known in the prior art. These same mechanisms insert the wafersinto the load-lock 70 via a second opening 76. The wafers are initiallyinserted into the load-lock 70 and the pressure within the load-lockreduced until the pressure inside the load-lock chamber 72 is the sameas the pressure inside the ion implantation chamber 22. A fluid tightdoor or hatch 80 is then opened to allow the wafers inside the load-lockto be transferred into the ion implantation chamber. Once the ionimplantation process is completed, the wafers are moved from the chamber22 back into the load-lock 70 and the load-lock is repressurized byallowing air to flow back into the chamber 72. A second door or hatch 82is opened and the treated wafers are withdrawn through the opening 76and moved to subsequent wafer handling and processing stations.

It is while the load-lock 70 is being evacuated and repressurized thatthere is a danger of dislodging particulate contaminants from the wallof the load-lock chamber 72. The flow rates of air entering and exitingthe chamber 72 are carefully monitored in accordance with the presentinvention. Acceptable flow rates are derived from theoreticalconsiderations of the forces the particulates clinging to the chamberwalls must experience before they are dislodged from those walls.

Theoretical Flow Rate Limit Calculation

The aerodynamic force acting on a particle due to a fluid flow is

    F.sub.aero =1/2{C.sub.d /C.sub.c }pu.sup.2 A.sub.p         (Equation 1)

where C_(d) is the drag coefficient, C_(c) is the Cunningham slip factor, p is the gas density, u is the local gas velocity at the particle andA_(p) is the cross sectional area of the particle. All of these factorsdepend solely on gas density (and therefore, pressure), local gasvelocity and particle size. The particles of concern are attached to thewalls of the load-lock chamber. The flow velocity at the wall is u=0because of the existence of what is referred to as the boundary layer.The velocity profile moving away from the wall and into the flow dependson the type of flow. The velocity gradient is more gradual in a laminarflow than in a turbulent flow. Equivalently, if one moves a smalldistance from the wall in a laminar flow, the velocity will be a muchsmall fraction of the free stream velocity than in the case of aturbulent flow. Of course, the free stream velocity in the turbulentflow may be greater. For a laminar flow the velocity a distance y fromthe wall will be

    v/u*=3/2(y/δ)-1/2(y/δ).sup.2                   (Equation 2)

where u* is the free stream velocity and δ is the boundary layerthickness. Typically, δ is the order of millimeters for the pressuresand flow velocities of interest here and decreases as (u*)⁻¹⁷⁸ .Combining with y=d, the particle diameter, gives

    F.sub.aero =G(d/P)p.sup.1/2 (u*).sup.3/2                   (Equation 3)

Here G(d,)) is weakly dependent on pressure.

In order for a particle to detach from the walls the aerodynamic forcemust at least large enough to slide the particle along the surface. Ifthe friction coefficient is k and the attachment force is F_(attach),then to detach a particle from the chamber walls

    F.sub.aero >k F.sub.attach                                 (Equation 4)

This attachment force will be independent of gas conditions. Thus, todetach particles

    (u*).sup.3/2 ˜k F.sub.attach /G(d,P)P.sup.1/2        (Equation 5)

Experiments have confirmed the correctness of this result. Theattachment force depends on a variety of particle parameters as well ason the properties of the wall. The same experiments have shown thatkF_(attach) is on the order of 5×10⁻⁶ dynes or greater. This is the onlyunknown in the above equation and allows the determination of thecritical velocity, u*_(crit) (P), such that particles will be detachedat greater velocities and not detached at lower velocities. u*_(crit)(P)/m, where m is the safety factor, is the optimum venting profile forair entering the load-lock or for air leaving during depressurization.Note that the critical velocity depends only on ambient pressure.

Returning to FIG. 2, as the load-lock 70 is pressurized a flowcontroller 110 is opened to allow air to enter the interior of thechamber 72 at a controlled flow rate. On an outlet side of the load-lock70, a second flow controller 112 i opened to allow air inside theload-lock 70 to be pumped out of the chamber 72.

On air inlet side the load-lock 70 includes a flow diffuser 114 whichachieves a uniform flow of gas across the area of the diffuser. Thediffuser 114 should be as large as practical compared to the chamberdimensions. In the disclosed embodiment, for example, the diffuser 114defines essentially one entire boundary wall for the load-lock chamber72. A flow passageway between the controller 110 and diffuser 114defines a tapered section 116 of increasing area.

On an outlet side of the load-lock 70, a second tapered passageway 120provides a transition between the relatively low flow rates inside theload-lock and the higher air flow rates in a passageway 122 leading tothe vacuum pump. Air velocity control is less critical on the outletside and a diffuser may not be necessary. If contamination exceedsacceptable limits, however, a second diffuser 121 shown in phantom inFIG. 2 can be utilized.

The two flow control units 110, 112 are adjusted by a programmedcontroller 130 (see FIG. 4). As air enters the chamber 72 the programmedcontroller 130 monitors an input from a pressure sensor 132 thatmonitors air pressure inside the load-lock 70. The theoreticaldiscussion above concerning the acceptable flow rates to avoiddetachment of the particles from the chamber walls is programmed intothe programmable controller 130 in the form of a look-up table. Thepressure monitored by the sensor 132 is correlated to a maximum flowrate as indicated in the graph (FIG. 6) schematically depicting thetheoretical considerations. This allows the controller 130 to calculatea critical velocity profile as the pressure within the load-lockchanges.

The two flow control units may be a mass flow control unit commerciallyavailable from many companies including, for example, SierraInstruments. The flow rate through a suitable flow control unit iscomputed in terms of volume of fluid per unit time, i.e. cubic feet persecond or the like. If the flow rate through the controller 100, forexample, is controlled, the velocity of air entering the chamber 72 pastthe diffuser 114 can be calculated by dividing this flow rate F by thecross-sectional area A of the diffuser, v=F/A. It is straight-forwardfor the controller 130 to correlate the measured pressure to a desiredflow rate based upon the maximum permitted velocity of the air in theregion of the diffuser 114 (see FIG. 6). A signal at an output 131 fromthe controller 130 adjusts the flow of air through the flow control 110to achieve an appropriate velocity V that avoid particle detachment.

The size of the chamber 72 and the flow rates needed to efficientlytransfer wafers require relatively large mass flow control units whichcan be expensive to utilize. In addition for depressurization commercialmass flow controllers are limited in conductance. An alternate controlscheme utilizes multiple pressure sensors 134-136 (FIG. 5) instead ofthe single pressure sensor 132. In this alternate embodiment, if thepressures on opposite sides of a valve 140 are measured, the valvesetting can be adjusted based upon the measured pressures. The flow rateof air passing through the valve 140 is equal to a constant Q, dependentupon the particular valve multiplied by the difference in pressure atthe two sensors 135, 136. In equation form

    F(flow)=Q(P.sub.1 -P.sub.2)                                (Equation 6)

In the FIG. 5 embodiment the controller 130 adjusts the setting of thevalve 140 by a control signal at an output 142 that opens and closes thevalve 140 depending on whether the gas velocity (v=F/A) is above orbelow a target gas velocity at the diffuser 114. Implementations otherother than FIGS. 4 and 5 will be apparent to those skilled in the art offlow control. The important feature of the invention is the control ofentering and exiting flow velocity as a function of chamber pressure.

Practice of the invention is not limited to situations in which aload-lock 70 is utilized. In FIG. 3, for example, the ion implantationchamber 22' is pressurized and evacuated directly so that air is allowedto enter the chamber 22' before wafers are either inserted into or takenout of the chamber 22' through the hatch 82'. The door or hatch 82' isthen closed and the chamber 22' is evacuated. The flow control rates ofboth pressurization and depressurization are monitored and adjustedusing flow control units 110', 112' in a manner similar to thetechniques discussed above in regard to FIG. 4 or FIG. 5. Variousmechanisms for inserting wafers directly into an ion implantationchamber such as the chamber 22' of FIG. 3 are known within the priorart. These insertion and withdrawal techniques, however, are consistentwith the monitoring of air flow as it is pumped from the chamber andallowed to enter the chamber during repressurization.

The present invention has been described with a degree of particularity.It is the intent, however, that the invention include all modificationsand alterations from the disclosed design within the spirit or scope ofthe appended claims.

I claim:
 1. A method for alternatively evacuating and pressurizing aprocess chamber while avoiding chamber contamination by dislodgingparticle contaminants comprising the steps of:a) defining a criticalmaximum air flow velocity which depends on sensed pressure and whichminimizes the dislodging of particle contaminants and which decreaseswith increasing air pressure; b) sensing pressure inside the chamber anddetermining an instantaneous air flow rate based upon the geometry ofthe chamber; and c) adjusting the flow rate of air entering or exitingthe chamber as the determined instantaneous air flow rate changes duringpressurization or evacuation to produce a velocity of air movementwithin the chamber below the critical maximum velocity.
 2. The method ofclaim 1 wherein the adjusting step is performed by setting a fluid flowcontroller to regulate the volume of air entering the chamber per unittime.
 3. The method of claim 1 wherein the adjusting step isaccomplished by measuring a pressure difference across a valve andchanging the valve setting to produce a desired pressure differencecorresponding to an appropriate air flow rate entering or exiting thechamber.
 4. The method of claim 1 including the additional steps ofinserting a semiconductor wafer int he chamber and impinging the waferwith ions from an ion source while the chamber is evacuated.